1. Field of the Invention
The present invention relates to a wire electric discharge machine which performs taper cutting.
2. Description of the Related Art
FIG. 15 shows taper cutting using a wire electric discharge machine in which machining is performed while a wire electrode supported by wire guide portions is extended obliquely with respect to a vertical direction 5 by moving the upper guide portion parallel to the plane of a table. As shown in FIG. 15, in conventional taper cutting, discharge machining is performed on a workpiece 3 mounted on the table 2 with the wire electrode 4 tilted with respect to the vertical direction 5.
Taper cutting in which machining is performed with the wire electrode 4 extended obliquely with respect to the vertical direction 5 has more technical problems than vertical cutting in which machining is performed with the wire electrode 4 extended vertically. Some of these problems are listed below.
1) Machining speed cannot be increased due to the difficulty of removal of machining sludge.
2) Frictional forces are exerted on the wire electrode by the wire guide portions to adversely affect surface roughness.
3) Support points of the wire electrode bent by the wire guide portions change depending on the accuracies of the shapes of the guides and the state of machining, thus making accurate machining difficult.
4) It is difficult to set machining conditions.
These are problems caused by obliquely extending the wire electrode, and have the property of becoming more significant with increasing taper angle.
Now, a conventional taper cutting technique will be described by taking as an example the machining of a workpiece in which the shape of a top surface is different from that of a bottom surface, such as shown in FIGS. 16A to 16C. A machining program for the workpiece is shown in FIG. 17. It should be noted that U/V commands specify the amounts of travel of the wire electrode 4 at the top surface of the workpiece in the X/Y directions.
Operations in accordance with the machining program shown in FIG. 17 will be described.
First, the command M15 enables the function of taper cutting.
The command G92X5.0000Y-35.0000 sets a coordinate system for the machining program and a machining start point (5,−35).
The command G90G01G42X10. moves the wire electrode 4 to the machining start point compensated by an offset.
The command Y-30. moves the workpiece-top side of the wire electrode 4 toward a point Pt0 and the workpiece-bottom side of the wire electrode 4 toward a point Pb0.
The command Y-20. moves the workpiece-top side of the wire electrode 4 toward a point Pt1 and the workpiece-bottom side of the wire electrode 4 toward a point Pb1.
The command X20.U-5.V5. moves the workpiece-top side of the wire electrode 4 toward a point Pt2 and the workpiece-bottom side of the wire electrode 4 toward a point Pb2 while tilting the wire.
The command Y-10.U0.V0. moves the workpiece-top side of the wire electrode 4 toward a point Pt3 and the workpiece-bottom side of the wire electrode 4 toward a point Pb3 while restoring the wire electrode 4 to an upright position.
The command X35. moves the wire electrode 4 to a point outside the workpiece 3.
The command G40Y-5. moves the wire electrode 4 toward a machining end point.
As described above, in the conventional taper cutting technique, machining is performed with the wire electrode 4 tilted while the tilt thereof is being changed during the machining. Accordingly, the aforementioned problems occur.
To cope with the problems, Japanese Patent Application Laid-Open No. 2012-254505 discloses a wire electric discharge machine including a table which has a plane formed by two orthogonal axes and a coordinate system and on which a workpiece is mounted, and a mount portion for mounting the workpiece obliquely with respect to the plane. The wire electric discharge machine performs discharge machining while moving a wire electrode and the workpiece relative to each other in accordance with a machining program for taper cutting. The coordinate system is converted by tilting the coordinate system by an angle equal to the tilt angle of the obliquely mounted workpiece with respect to the plane. Based on the converted coordinate system, specified machining values commanded by the machining program are compensated.
The wire electric discharge machine disclosed in Japanese Patent Application Laid-Open No. 2012-254505 deals with the same problems as the present invention, and reduces the tilt of a wire electrode by tilting a workpiece at a fixed angle during machining, thus trying to solve the problems of taper cutting. However, the wire electrode is actually tilted at a slight angle. Accordingly, the aforementioned problems caused by extending the wire electrode obliquely with respect to the vertical direction cannot be well avoided.